This invention relates to an encoder apparatus, in particular a position measurement encoder apparatus comprising a scale and a readhead which are moveable relative to each other.
As is well known, a position measurement encoder apparatus typically comprises a scale having a series of features which the readhead can read to determine and measure relative position (and its derivatives such as velocity and/or acceleration). Encoders are typically categorised as being either incremental or absolute. A scale for an incremental encoder (such as the TONiC™ encoder available from Renishaw plc and described in more detail below) comprises a series of generally periodic features which the readhead detects to determine a relative position and movement of the scale and readhead. One or more reference marks can be provided on the scale to provide reference positions from which the relative position of the scale and readhead can be counted. A scale for an absolute encoder (such as the RESOLUTE™ encoder available from Renishaw plc and described in more detail below) comprises features defining unique positions along the scale length, for instance a series of unique absolute positions, and can enable a readhead to determine its absolute position on start-up without requiring any relative motion.